Preheat the oven to 200ºC, fan 180ºC, gas 6.
Boil a large pan of salted water, add the leeks and cook for 5 minutes.
Drain the leeks and transfer to a buttered oven dish and arrange the salmon slices on top.
In a small pan, melt the butter. Add the flour and stir continuously for 2 minutes until lightly golden.
Remove from the heat and slowly whisk in the milk to make a smooth sauce. Turn the heat back on and bring up to a simmer. Stir continuously for 2-3 minutes.
Remove from the heat and beat in the egg yolks, mustard and season to taste. Pour the sauce over the leeks and salmon and sprinkle with grated cheese.
Cook in the oven for 10 minutes until the cheese is bubbling and golden. Serve immediately.
Recipe courtesy of Great British Chefs and Nathan Outlaw.